The Whistleblower, based on a true tale of forced prostitution in Bosnia, is not the first movie I would have rushed out to see. Sex-slavery films such as Lilya 4-Ever (2003) and Trade (2007) are a tragic mini-genre, filled with bruised, trembling girls trapped in windowless rooms, waiting to be taken to work or a beating. Help rarely shows up, unless it’s a Liam Neeson movie (2008’s Taken).

But the prospect of Oscar winner Rachel Weisz as a troubled Nebraska cop who stumbles onto corruption in Bosnia offered enough incentive to get me into a chair. This is the farthest out of her range Weisz has gone since 2001, when she played a Russian militia member in the Stalingrad-siege movie Enemy at the Gates. And what’s a Nebraska cop doing in Bosnia anyway? Mending her financial woes back home by taking a contract job as a United Nations peacekeeper. Weisz plays Kathryn Bolkovac, who arrived in Bosnia in 1999 and discovered that other U.N. contractors knew about sex-trafficking rings, and some of them were actually assisting in transporting and selling the girls.

Bolkovac raised the issue, got fired and went public. Hers is a compelling story (it’s also a book called The Whistleblower, written by Bolkovac with Cari Lynn), though not one that was widely covered in the American media. Knowing little about Bolkovac in advance adds to the grim suspense of director and co-writer Larysa Kondracki’s narrative.

Before we meet Kathy Bolkovac, there’s a glimpse of some of the stolen girls. Ukrainian teen Raya (Roxana Condurache) is lured into slavery by one of her friends, who believes they’re on their way to respectable hotel jobs. In movies like this, the minute you see a girl posing for a passport picture, it’s as if a guillotine is being lowered. By the time Kathy encounters Raya during a raid, it has been: she’s beaten, bloodied and barely able to speak.

Bosnia to that point has mostly been good for Kathy. She encounters some unsurprising sexism, but hooks up with a hot Danish peacekeeper (Nikolaj Lie Kaas) and is promoted to a job with the gender affairs office of the UN’s High Commission for Human Rights. Her boss, Madeleine Rees, is played by Vanessa Redgrave in one of those grave performances that suggest a quiet power — the kind she does so well.

But power is in depressingly short supply for Kathy and her colleagues, as she discovers. “I was dumb enough to think I’d be joining some elite police officers here,” she says ruefully; plunging ahead with an investigation, she realizes how fleeting a chance she has at justice. Kathy’s impotence counters the impulse to view The Whistleblower as another example of the Western hero sweeping into save the day. The deeper I got into the story, the less confidence I had in any kind of happy outcome, but the more I saw of Raya and her frantic mother, the more I hoped for it.

The bad guys are everywhere, and at times they’re hard to distinguish from each other. (“Where are we going, Columbo?” asks one mid-level scumbag.) Monica Bellucci gives a strangely sleepy performance as an unhelpful government agent whose motives are unclear. But Condurache and Weisz’s performances are steadily engaging. Weisz is a dazzling woman, but her beauty is barely noticeable in this role; her character’s integrity and her mounting anger grab all the attention. In one scene Kathy finally confronts what she’s up against and starts to cry. They are tears of rage, and the most powerful I’ve seen this year.

Mary Pols has been reviewing film and occasionally books for TIME since 2009. She has degrees from Duke University, where she studied art history, and the University of California Berkeley’s School of Journalism. She is the author of a memoir about untraditional parenthood. It was made into a short-lived television comedy where everything turned out traditionally.

Accidentally on Purpose: The True Tale of a Happy Single Mother is available for nearly nothing at various online bookstores and from boxes in Pols' barn.